Reflux, Distillation & Boiling Point Determination
Heating under reflux
- Organic reactions often occur slowly at room temperature
- Therefore, organic reactions can be completed by heating under reflux to produce an organic liquid
- This allows the mixture to react as fully as possible without the loss of any reactants, products or solvent
- In distillation, you are trying to separate a chemical or product from a mixture
- When heating under reflux, you aim to keep all the chemicals inside the reaction vessel
The Heating under Reflux Process:
- Example reactions where heating under reflux could be used include:
- The production of a carboxylic acid from a primary alcohol using acidified potassium dichromate
- The production of an ester from an alcohol and acid in the presence of an acid catalyst
- The reaction mixture is placed into a pear-shaped or round bottomed flask
- Anti-bumping granules are, again, added to promote smooth boiling
- The flask is placed in a heating mantle or it can be immersed in a water bath for heating
- Quickfit apparatus is then set up with the condenser clamped vertically in place
- The joints of the Quickfit apparatus are commonly greased as with distillation
- A steady and constant stream of water passes through the condenser in a 'water jacket' - it enters at the bottom of the condenser and the drainage pipe removes the water from the top of the condenser
- The water is heated and the reaction mixture allowed to boil
- The heated is stopped and the mixture allowed to cool back to room temperature
The preparation of ethyl ethanoate involves heating under reflux for about 15 minutes
Distillation
- Distillation is a common practical completed in organic chemistry
- Distillation is used as there are times that a reaction does not go to completion or there are other chemicals produced as well as the desired product
- Distillation allows you to separate compounds by their boiling point
- Chemicals with the lowest boiling point will distill first
- One of the most common distillation practicals is the oxidation of primary and secondary alcohol to aldehydes and ketones
The Distillation Process:
- To produce an aldehyde from a primary alcohol, a reaction mixture containing the primary alcohol and acidified potassium dichromate solution is placed into a pear-shaped or round bottomed flask
- Anti-bumping granules are added to promote smooth boiling
- Quickfit apparatus is then set up, including a still head and condenser connected to the side
- The joints of the Quickfit apparatus are often have a thin layer of silicon grease smeared over them to give a better seal as well as to make it easier to disassemble the equipment afterwards
- A Quickfit thermometer can be used, with the thermometer bulb sitting where the vapours will pass into the condenser
- A steady and constant stream of water passes through the condenser in a 'water jacket' - it enters at the bottom of the condenser and the drainage pipe removes the water from the top of the condenser
Heating under Distillation Apparatus
- The reaction mixture is heated until it boils using a heating mantle
- Electric heating mantles are used for this because the temperature can be controlled, and because you are using chemicals which are flammable
- The distillate which forms in the condenser drips directly into a receiving vessel
- The distillate which should be collected, is that which is given off at +/- 2 oC of the boiling point of the desired product
- Some distillate may be given off below this temperature - this needs to be discarded and a clean vessel used to collect the desired product
- Stop collecting the distillate if the temperature rises above +/- 2oC of the boiling point of the desired product
- The aldehyde product has a lower boiling point than the alcohol (since it has lost the H-bonding) so it can be distilled off as soon as it forms
Examiner Tip
- These practicals give you the opportunity to discuss:
- The use of an electric heating mantles and water baths rather than a Bunsen burner
- The choice and setup of laboratory apparatus
- Health and safety considerations including the careful handling of different liquids, including those which are corrosive, irritant, flammable and toxic
Boiling point determination
- The boiling point of a liquid is indicative of its purity and identity
- Boiling point is determined by distillation
- The sample is gently heated until it boils and this temperature is recorded
- This boiling point can then be compared against literature / database values
- If the sample contains impurities:
- The boiling point may appear higher than the literature / database values
- The sample may boil over a range of temperatures instead of at a single temperature